Electronic switching systems are well known in the prior art. However, in spite of the many advantages of electronic systems over the widely used electro-mechanical systems, electro-mechanical systems comprise approximately 90% of all of the telephone switching systems used in the world. The reason for this is that it is more economical for telephone companies to extend existing installations rather than replace such installations with completely new electronic units. Moreover, in the vast majority of telephone switching installations, the electro-mechanical switching units are functioning satisfactorily and are not fully depreciated.
It is an object of the present invention to provide a telephone selector switching unit which utilizes electronic control circuits and which is sufficiently compatible with existing step-by-step electro-mechanical switching arrangements so as to be capable of replacing such arrangements or be added on to extend system capability.
It is a further object of the present invention that such electronically controlled selector unit be capable of being part of a totally new switching system.
Most telephone systems in common use employ three or four wire systems. As a general rule, selector units for the switching arrangement must be designed for either three wire or four wire use and only considerable modification renders a selector designed for one system adaptable for the other.
It is another object of the present invention to provide a selector unit which is adaptable by a simple strapping of terminals to operate efficiently in either a three wire or four wire mode.
Progressive or step-by-step switching in telephone systems utilizes a series of selectors in which gross motion switches respond to each dial pulse by moving in an appropriate direction. Each selector makes a one out of ten selection with access after each selection to ten links in the next stage. This progressive switching approach is distinguished from the common control switching approach wherein a common control circuit determines whether a called line is busy before establishing a connection and determines a switching path according to traffic requirements rather than being dependent upon directory numbers.
It is an object of the present invention to utilize common control techniques within a progressive selector unit.